In disk-type random access magnetic memories where data is recorded in concentric circular tracks on the surfaces of disks, it is a continuing aim to accurately align a magnetic transducer with a desired track. The degree of accuracy with which the transducer can be positioned determines the spacing necessary between the center of adjacent tracks and thereby largely influences the storage efficiency, i.e, number of characters per unit of area of the memory. In an attempt to increase the accuracy of alignment, servo systems of various types have been proposed for servoing the transducer onto the tracks. These systems have generally employed positioning information in the form of servo signals interspersed with the data in the recording surface (called sector servo) or reference patterns permanently recorded on a disk surface. Those systems which employ a separate dedicated servo surface have required a servo transducer to read the positioning information on this surface and one or more separate data transducers ganged thereto. This inherently militates against high storage efficiencies because of the stack-up of mechanical tolerances in the ganged transducers. In systems using sector servo, the same head can be used to read both the data and the interspersed servo information, but because of the high frequency sampling required for accurate positioning, such systems do not provide the desired accuracy unless a considerable portion of the available disk surface is given over to the storage of positioning information.
As an alternate to servo systems employing separate servo sectors or servo surfaces, the so-called buried servo has been employed. In one form of such a servo, a disk having two magnetic layers is used, the lower layer containing a recorded servo pattern and the upper layer containing the recorded data, with the lower servo layer having a considerably higher coercivity than the upper layer so as to avoid erasure of the servo information while recording data on the upper surface.
In an alternate embodiment of buried servo, only a single magnetic layer is employed and the servo information is recorded thereon with a special recording head. The top portion of this layer is erased to erase the servo information in this top portion. Data is then recorded in this top portion by a thin film head which does not affect the servo information recorded in the bottom portion of the magnetic layer.
For continuous servoing from such prerecorded servo information with only one recording head, which is used simultaneously for reading the servo and for data writing or data reading, the system must have three modes of operation. These are reading servo while (1) reading data, or (2) while writing data or (3) while neither reading nor writing data. Clearly the second mode of operation, reading servo while writing data, presents the most serious problem. To prevent the write signal from appearing in the servo readback channel, prior systems have used write and servo filters that have a very high stop band attenuation, only little pass band attenuation and consequently very steep filter slopes.